1. Field of the Invention
The present disclosure relates generally to computer networking. More particularly, the present disclosure relates to spanning tree protocols.
2. Description of the Background Art
Spanning tree protocol (STP) and its variants, including rapid spanning tree protocol (RSTP) and multiple-instance spanning tree protocol (MSTP), are link management protocols that prevent undesirable loops in a network while allowing for path redundancy. Undesirable loops occur when there are multiple active paths between stations. If a loop exists, a switch or bridge may see stations appearing on more than one link of the switch. This can confuse the forwarding algorithm, allowing duplicate frames to be forwarded. Further, multicast traffic entering the loop multiplies, but never leaves the loop, eventually bringing the network down.
STP computes a tree that spans all switches in an extended network and forces select redundant data paths into a standby or blocked state. If one segment of the network becomes unreachable, STP can re-establish connectivity to that segment by activating a standby path. In order to accomplish this, one switch (bridge) is designated as the root of the spanning tree. STP is a distributed protocol and relies on the exchange of messages between neighbors to compute and implement the spanning tree. When there are changes in the network due to failure or equipment upgrades, the protocol needs to recompute the tree. During such times, the forwarding of traffic may be disrupted for a period of time, sometimes referred to as the convergence time of the protocol, which can range from milliseconds to seconds.
The traditional spanning tree protocol (IEEE 802.1d) is limited to only one path through the spanning tree environment. Multiple-instance spanning tree protocol (MSTP) (IEEE 802.1.s) addresses this limitation. MSTP allows multiple spanning tree instances, each instance providing a different path through the multiple-instance spanning tree (MST) region. With MSTP, numerous virtual local area networks (VLANs) may be mapped to various MSTP instances.